Oil field operators drill boreholes into subsurface reservoirs to recover oil and other hydrocarbons. If the reservoir has been partially produced or if the oil is particularly viscous, the oil field operators will often inject water or other fluids into the reservoir via secondary wells to encourage the oil to move to the primary (“production”) wells and thence to the surface.
Information about the geologic formations and about reservoir characteristics promotes efficient development and management of hydrocarbon resources. Reservoir characteristics include, among others, resistivity of the geologic formation containing hydrocarbons. The resistivity of geologic formations is generally related to porosity, permeability, and fluid content of the reservoir. Because hydrocarbons are generally electrically insulating and most formation water is electrically conductive, formation resistivity (or conductivity) measurements are a valuable tool in determining the hydrocarbon content of reservoirs. Moreover, formation resistivity measurements may be used to monitor changes in reservoir hydrocarbon content during production of hydrocarbons.
Generally, formation resistivity is sensitive to fluid saturation. The ability to remotely determine and monitor formation resistivity is of direct relevance to reservoir monitoring, particularly for enhanced recovery with water-flooding and/or carbon dioxide injection. Accordingly, there is continued interest in the development of permanent sensing systems.